Latices of various polymers are useful for a wide range of applications. In recent years, rubber latices have been widely employed for the manufacture of rubber-reinforced plastics such as impact styrene and ABS materials. Although rubbers may be mechanically admixed with such polymers, greatly improved results are obtained by providing chemical adhesion between the rubber phase and the matrix by grafting at least a portion of the matrix polymer onto the rubber particles. It has also been noted that the impact strength of the rubber-modified compositions at times is dependent upon the size of the rubber particles dispersed therein within certain limits so that there has been interest in increasing the size of the rubber particles obtained in conventional latex polymerization processes.
In U.S. Pat. No. 3,558,541 to William O. Dalton, issuing on Jan. 26, 1971, there is disclosed an improved batch process for agglomerating latices based upon the use of acid anhydrides and latices containing organic acid salts as the emulsifying agents. The acid anhydride destroys the emulsifying agent because of its higher ionization constant and the unstabilized latex permits the particles to collide and agglomerate into large particles which are subsequently stabilized.
This technique has proven highly effective in obtaining latices with particle sizes varying with the polymer and ranging up to about 1 micron. However, there has remained a desire to obtain even larger particles with some latices and a desire to reduce the amount of the relatively expensive acid anhydride and time required to obtain particles of a given size.
In U.S. Pat. No. 3,551,370 to William O. Dalton issuing on Dec. 29, 1970, there is disclosed an improved batch process for agglomerating aqueous latices of a suitable polymer wherein an inorganic electrolyte is added to the latex prior to admixture with a water-soluble acid anhydride. Such batch process handling large volumes of latex and anhydride solution, requires mixing in large stirred tank, hence has considerable difficulty in uniformly mixing said materials, time-wise, before the pH drops too far in localized areas and the mixing can then cause serious coagulation and nonuniformities batch to batch as well as loss of coagulated latex.
It is an object of the present invention to provide a novel continuous process for agglomerating relatively small particles in an aqueous latex into larger particles with good control over the range of particles produced and which affords improved economy and production rates.
It is also an object to provide such a continuous process which may be used with various latices of polymers having a composition providing a surface permitting fusion or bonding of colliding particles.
A further object is to provide such a continuous process which is particularly advantageous for producing large rubber particles useful as a reinforcing agent in various polymeric matrices.
A further objective is to overcome the problem of batch mixing large volumes of the agglomerating solution uniformly with large volumes of latex quickly, at reasonable temperatures for agglomeration, without creating sufficient shear in said admixture to cause serious coagulation. The present process provides continuous fast, uniform mixing of the feed streams in relatively small mixing zones followed by flow to a laminar-flow zone with low shear for controlled agglomeration.